1. Field of the Invention
The present invention relates to a private branch exchange and a line exchange method and, more particularly, to a private branch exchange which accommodates a plurality of extension terminals and which is arranged to effect line exchanges by specifying a destination terminal from among the extension terminals on the basis of a signal transmitted from a transmitting side, as well as a line exchange method for such a private branch exchange.
2. Description of the Related Art
A general private branch exchange serves the function of transmitting a call to an extension terminal which is specified by a calling party from among a plurality extension terminals. For example, an ISDN which is a typical public digital communications network is capable of transmitting not only a destination line number but a subaddress by dialing. Accordingly, with ISDN, it is possible to specify a particular extension of a private branch exchange connected to the network and transmit a call directly to a corresponding extension terminal.
However, in this example, if the received call contains no called subaddress or there is no terminal corresponding to the received called subaddress, it is possible to specify a particular extension terminal to be called. Accordingly, it is necessary to ring or activate all the extension terminals corresponding to the line number, thus leading to the following problems.
(1) The probability that an unwanted person or terminal responding is high. As a result, a respondent or a responding terminal may respond to unnecessary communication which is not economical.
(2) If information is transmitted from one terminal to another terminal such as facsimile machines, an unrelated terminal may respond to the terminal at the transmitting side. As a result, secrets may leak or documents may be lost.
(3) Even if a predetermined party is informed of the subaddress at the receiving side in advance, an unwanted call from any party other than the predetermined party must be received. Accordingly, economic savings and traffic efficiency deteriorate.